Objective: Aim of this study was to analysis the ankle moment-angle relation and its characteristics at different gait speeds and phases for using in prosthetic ankle-foot design.

Materials & Methods: This was a cross-sectional analytic study in which 20 participants were chosen with assessable sampling method. Gait analysis at different speeds was performed with two force-plates and five high speed cameras. Ankle characteristics including quasi-stiffness and work were estimated at three periods of stance phase based on moment-angle loop. Mean differences were analyzed with repeated measure ANOVA and regression analysis was performed to fit the linear model to the data.

Results: Moment-angle curve was clockwise at slower speeds and turned counter-clockwise while speed increased. The best fitted line to the work-walking speed curve had R2=0.81. There were significant differences in quasi-stiffness between fast and normal speed and slower speeds (P<0.001).

Conclusion: Moment-angle curve displayed ankle passive function in slower speeds and active function in faster speeds. Results showed ankle characteristics varied at different gait phases and speeds. Thus, at slower speeds a spring-damper model with regulable damping would simulate the ankle function. However, at higher speeds the active elements also should be considered.